Derby Talk

Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories

Our club has had a RaceMaster IV finish line timer for many years, and there have been several problems with it over the years. The last 2 years, I managed to put some time aside to figure these out, and I'd like to share my findings here, in case others are having similar issues.

Whisker Switch: I've seen several posts about people complaining about these, and I agree. The whisker tends to get bent throughout the race, making the starting trigger unreliable. I replaced the starting line microswitch with a more robust lever actuated microswitch. I bought mine at our local Frys Electronics store, part number VT16031C2, but you can use any microswitch with a suitably robust lever.

Timer Occasionally Would Not Start: All switches have what we call "bounce", where the internal switch contacts would repeatedly make and break connections during the time that the switch is actually activated. The RaceMaster IV circuit includes an RC filter with the intent of debouncing the switch, and cleaning up the signal before it gets to the microprocessor. However, the filter components were not calculated to account for a bounce time of more than about 200us. The switch I was using had a bounce time of up to 2ms. To fix this, I changed C15 to a 4.7uF, 10V, 1206 capacitor. This cleaned up the signal, so that the processor saw a single, perfect rising edge when the starting line trigger switch activated. As a result, the processor does not miss a starting line activation any more. I also replaced C16 with the same size capacitor. C16 is used in the debounce filter for the reset button.

Timer Automatically Resets Seconds After Starting: Upon first powerup, the timer would reset very quickly after starting, often less than a few seconds. The display would read FOO HOO, and then reset to all 0s. This was nowhere near long enough to run a race. As the electronics were powered on for longer periods of time, the timer would run longer before resetting. It would also run longer if one of the finish detectors was triggered. Eventually, it ran long enough for us to run races. It turns out that the input power supply, as it was used in the mezzanine circuit board where the timers reside, was unstable. Over time, the stability would improve. This was likely due to the extensive use of aluminum electrolytic capacitors, which have a tendancy to dry out over time, and lose their performance. I added a 330uF 35V aluminum electrolytic capacitor to the input voltage (because I happened to have a bunch of those lying around), which stabilized the power, and allowed the timer to run flawlessly. I would recommend a couple of 100uF 35V or higher tantalum capacitors if you can find them, as those will tend to last a lot longer than the aluminum electolytics. Pay attention to the polarity of both aluminum electrolytic and tantalum capacitors.

Hopefully, that info will be helpful to others with that timer and have some electronics experience. Though, with the cost of that timer compared to competing timers, it is a shame that such measures would need to be done to get it working reliably.

Thank you jzelno for the C15 & C16 replacement clues. For the start switch I'm wondering if a gold flashed crosspoint contact type switch would also increase reliability?
For the power supply problem you say to use 100uF 35V electrolytics, where are these placed? Do they replace the electrolytic on the front of the main timer board? On our RaceMaster IV it uses a 9VDC wall wart for the power supply.

Gold flashed contacts are certainly more reliable than standard tin plated contacts. However, the use that these start switches see isn't all that much that you would notice.

I'm not quite sure I know what a "crosspoint contact type" switch is, though I don't believe that the switch mechanism will make a difference. All you really need is a single pole single throw (SPST) switch, though I can't remember if it's normally open (NO) or normally closed (NC) (I'm writing this from memory, as I don't have the unit in front of me). You can also use a single pole double throw (SPDT) switch, and use the correct terminals. The actuator style on your start switch will depend on how your starting gate is implemented. A lever actuated microswitch should work well for most applications, but it's important that you have a sturdy lever on the switch. The originally supplied microswitch had a "whisker" that is really just a thin, flexible wire. If your start gate "slams" into the whisker, it can easily bend it out of shape, causing the switch to fail to trigger. If you can share a link to an example of a crosspoint contact type switch, I can tell you whether or not I think it would help.

For the 100uF capacitor (one should do it), you need to put this on the circuit board. I believe I put mine across the input pins of the 5V regulator. I don't recall the details or reference designator, but the regulator is an LM7805 style regulator, in either a TO-220 or TO-263 package. Put the capacitor between input and ground. We also have a wall wart supplying power, but it's pretty noisy. You really need the capacitor near the regulator... if you put it in the wall wart (or if you replace the wall wart with something else), then the long power supply wire can pick up noise making the capacitor ineffective. You don't need to replace any existing capacitors. In this area, more is better (as long as you can fit them, though one should be sufficient). Also, if you can find tantalum capacitors, these will be better than aluminum electrolytics. You can find everything you need at www.digikey.com or www.mouser.com.

Sorry I don't have more exact details. If you do need more details, let me know, and I'll see if I can get access to the unit I modified to take some pictures and more exact notes.

Thanks for all this information, the switch contact needed is NO. I use Allied, www.alliedelec.com and the switch I'll order is http://www.alliedelec.com/zf-electronic ... /70461913/. The D40 and D41 versions of this (see the data sheet) have the crosspoint contacts, the fact that the current rating is very low (Switch; Micro; SPDT; NO/NC; Straight Lever Actuator; Gold Contacts; 100mA;) means it is designed to switch "signal" circuits. A similar switch with higher power ratings, 5, 10 and 15 amps are usually silver or tin plated brass contacts and are not as reliable as gold for signal levels. The "cross" is like two knife edges coming together at 90°, it is supposed to reduce bounce and help with timing and accuracy, we used them in high speed robotics, I even found some in a reed switch configuration.

I got involved with setting up the track two days ago, decided to "fix" it and could not get the switches I wanted locally, that is when I found this post of yours, so I was thankful for that. We ran the races last night with only two re-starts necessary! So it did trigger most of the time.

For the capacitor my choice for placement was going to be at the regulator, so thanks for the clarification. Once again, thanks for all your insight.

Well, it seems that the manufacturer of the RaceMaster timer is no longer in business. I heard that the owner passed away. That timer is no longer listed in the Awana or Gospel Publishing House catalogs and the phone number has been disconnected. That, of course, does not bode well for those that need support for their timer.